1. Field of the Invention
The present invention relates to a method of manufacturing a bonded wafer. More specifically, the present invention relates to a method of manufacturing a bonded wafer having an oxide film with an uniform thickness of about a few nanometers.
2. Discussion of the Background
When bonding two silicon wafers through an oxide film to manufacture a bonded wafer, the oxide film is normally formed by heat treatment under an oxygen gas atmosphere and the formed oxide film usually has a thickness of equal to or greater than 100 nm. Such a manufacturing method is described, for example, in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 11-26336, which is expressly incorporated herein by reference in its entirety.
However, the thickness of an oxide film that functions as a BOX layer has become thinner with the miniaturization of devices and reduction of electric power consumption. Furthermore, directly bonded wafers without a BOX layer have been developed. However, it is difficult to uniformly form an oxide film with a thickness of about a few nanometers using conventional heat treatment methods. In particular, when a batch-type heat treatment furnace is employed, a load lock-type transfer equipment, an equipment for providing an ozone gas, and the like are required, resulting in increase in manufacturing costs.
An oxide film formed on a silicon surface by heat treatment is made of silicon oxide, SiO2. Since the oxide film made of SiO2 is a stable film, such an oxide film does not diffuse outward and thus it is condensed or becomes smaller during subsequent manufacturing process (heat treatment process). Finally, it remains as local oxide on an interface (that may be referred to as “bonding interface”, hereinafter) through which two silicon wafers are bonded. As a result, there may be a risk that problems such as failure of device formation and deteriorated characteristics occur. Such problems are caused by an oxide that is present on the bonding interface has not diffused completely outward and remains on the interface.
The aforementioned heat treatment is conducted, for example, to increase the strength of the interface through which two silicon wafers are bonded, to increase the surface flatness, and the like. For example, in the heat treatment conducted for increasing the strength of bonding interface, the bonded wafer is kept at a temperature of equal to or higher than 1,100° C. for equal to or longer than 1 hour. Examples of the heat treatment for increasing the surface flatness include a heat treatment conducted with argon, hydrogen gas, and the like at a temperature of equal to or higher than 1,100° C. Such heat treatment is described, for example, Japanese Unexamined Patent Publication (KOKAI) No. 2005-197524 or English language family member US Patent Application Publication No. 2007/0190737 A1, which are expressly incorporated herein by reference in their entirety.
When two wafers are bonded through no oxide film, the wafers are bonded following usual cleaning such as SCl. However, in that case, an oxide film with a thickness of equal to or greater than 2 nm is formed on the silicon wafer after conducting conventional cleaning, and local oxide remains on the bonding interface by the heat treatment during bonding process.